EL panel

ABSTRACT

In an EL panel comprising EL phosphor thin films of three types which emit red, green and blue light, respectively, the EL phosphor thin films of three types commonly and essentially contain Eu as a luminescence center. The EL panel comprising such phosphor thin films eliminates a need for RGB phosphor filters, has a satisfactory color purity and is best suited for driving RGB in full-color EL display.

TECHNICAL FIELD

[0001] This invention relates to an inorganic electroluminescent (EL)panel, and more particularly, to a full color EL panel having lightemitting layers for the three primaries RGB.

BACKGROUND OF THE INVENTION

[0002] In the recent years, active research works have been made onthin-film EL devices as small or large-size, lightweight flat paneldisplays. A monochromatic thin-film EL display using a phosphor thinfilm of manganese-doped zinc sulfide capable of emitting yellowishorange light has already become commercially practical as a dualinsulated structure using thin-film insulating layers 2 and 4 as shownin FIG. 2. In FIG. 2, a predetermined pattern of lower electrodes 5 isformed on a substrate 1, and a first insulating layer 2 is formed on thelower electrode-bearing substrate 1. On the first insulating layer 2, alight-emitting layer 3 and a second insulating layer 4 are successivelyformed. On the second insulating layer 4, a predetermined pattern ofupper electrodes 6 is formed so as to construct a matrix circuit withthe lower electrodes 5.

[0003] Thin-film EL displays must display images in color in order thatthey find use as computer, TV and similar monitors. Thin-film ELdisplays using sulfide phosphor thin films are fully reliable andresistant to environment, but at present regarded unsuitable as colordisplays because EL phosphors required to emit light in the primaries ofred, green and blue have poor characteristics. Engineers continuedresearch on SrS:Ce (using SrS as a matrix material and Ce as aluminescence center) and ZnS:Tm as a candidate for the bluelight-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the redlight-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for thegreen light-emitting phosphor.

[0004] These phosphor thin films capable of emitting light in theprimaries of red, green and blue suffer from problems of emissionluminance, emission efficiency and color purity. Thus color EL panelshave not reached the commercial stage. Referring to the blue color amongothers, a relatively high luminance is achieved using SrS:Ce. However,its luminance is still short as the blue color for full-color displayand its chromaticity is shifted toward green. There is a desire to havea better blue light-emitting layer.

[0005] To solve the above problem, thiogallate and thioaluminate baseblue phosphors such as SrGa₂S₄:Ce, CaGa₂S₄:Ce, and BaAl₂S₄:Eu weredeveloped as described in JP-A 7-122364, JP-A 8-134440, CommunicationSociety Technical Report, EID 98-113, pp. 19-24, and Jpn. J. Appl.Phys., Vol. 38 (1999), pp. L1291-1292. These thiogallate base phosphorsare satisfactory in color purity, but suffer from a low luminance andespecially, difficulty to form a thin film of uniform compositionbecause of the multi-component composition. It is believed that thinfilms of quality are not obtainable because of poor crystallinityresulting from inconvenient composition control, formation of defectsresulting from sulfur removal, and admittance of impurities; and thesefactors lead to a failure to increase the luminance. In particular,thioaluminate base phosphors are quite difficult to control theircomposition.

[0006] In order to develop practical full-color EL panels, phosphormaterials capable of providing blue, green and red phosphors in aconsistent manner and at a low cost are necessary. Since matrixmaterials of phosphor thin films and luminescence center materialsindividually have differing chemical or physical properties as describedabove, light-emitting performance differs depending on the identity ofthe phosphor thin film. Especially, the response speed and afterglow oflight emission differ between different luminescence centers. To driveblue, green and red pixels, a burning method matching with each color isnecessary.

[0007] Moreover, the EL spectra of the aforementioned blue, green andred EL phosphor thin films are all broad. When they are used in afull-color EL panel, RGB necessary as the panel must be cut out of theEL spectra of the EL phosphor thin films using filters. Use of filterscomplicates the manufacture process and, still worse, brings about alowering of luminance. When RGB is taken out through filters, theluminance of blue, green and red EL phosphor thin films suffers a lossof 10 to 50% so that the luminance is reduced below the practicallyacceptable level.

[0008] To solve the above-discussed problem, there remains a need forred, green and blue phosphor thin film materials capable of emittinglight of a sufficient color purity to eliminate a need for filters andat a high luminance, as well as an EL panel in which an identicalluminescence center is used in red, green and blue phosphor thin filmsso that they have the same response speed and afterglow of lightemission, allowing a common drive method to be used to drive blue, greenand red pixels, without a need for a separate burning method matchingwith each color.

SUMMARY OF THE INVENTION

[0009] An object of the invention is to provide an EL panel comprisingphosphor thin films eliminating a need for RGB phosphor filters, havinga satisfactory color purity and best suited for driving RGB infull-color EL display.

[0010] This and other objects are attained by the present inventionwhich provides an EL panel comprising EL phosphor thin films of threetypes which emit red, green and blue light, respectively, the ELphosphor thin films of three types commonly and essentially containingeuropium as a luminescence center.

[0011] In one preferred embodiment, the EL phosphor thin films of threetypes have the compositional formula:

A_(x)B_(y)O_(z)S_(w):R

[0012] wherein A is at least one element selected from the groupconsisting of Mg, Ca, Sr, Ba and rare earth elements, B is at least oneelement selected from the group consisting of Al, Ga and In, x is in therange of 0 to 5, y is in the range of 0 to 15, z is in the range of 0 to30, w is in the range of 0 to 30, and R is an element serving as theluminescence center and essentially containing europium.

[0013] In a further preferred embodiment, the EL phosphor thin filmwhich emits red light is made of a matrix material comprising analkaline earth sulfide, the EL phosphor thin film which emits greenlight is made of a matrix material comprising an alkaline earththiogallate, and the EL phosphor thin film which emits blue light ismade of a matrix material comprising an alkaline earth thioaluminate.Typically, the alkaline earth sulfide is calcium sulfide; the alkalineearth thiogallate is strontium thiogallate; and the alkaline earththioaluminate is barium thioaluminate.

[0014] In a further preferred embodiment, the EL phosphor thin films ofthree types which emit red, green and blue light, respectively, eachcomprise an oxysulfide obtained by incorporating oxygen in at least onecompound selected from the group consisting of an alkaline earthsulfide, alkaline earth thioaluminate, alkaline earth thiogallate, andalkaline earth thioindate. The molar ratio of oxygen element to sulfurelement in the oxysulfide, as expressed by O/(S+O), is in the rangebetween 0.01 and 0.85.

[0015] In another embodiment, any one of the EL phosphor thin films ofthree types which emit red, green and blue light, respectively, is madeof an oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic cross-sectional view showing an exemplaryconstruction of manufacturing apparatus to which the invention isapplicable.

[0017]FIG. 2 is a partially cross-sectional, perspective view showing anexemplary construction of an inorganic EL device according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The EL panel of the invention has EL phosphor thin films of threetypes which emit red, green and blue light, respectively. The elementadded as a luminescence center to the EL phosphor thin films of threetypes essentially contains at least europium (Eu) commonly to the threetypes.

[0019] The EL phosphor thin films of three types which emit red, greenand blue light, respectively, each are made of a matrix materialselected from among an alkaline earth sulfide, alkaline earth oxide,alkaline earth thioaluminate, alkaline earth aluminate, alkaline earththiogallate, alkaline earth gallate, alkaline earth indate, and alkalineearth thioindate, to which at least Eu is added as the luminescencecenter.

[0020] These EL phosphor thin films emit red, green and blue light of asufficient color purity to eliminate a need for filters and to a highluminance. Since Eu is used as a common luminescence center in red,green and blue phosphor thin films so that they have the same responsespeed and afterglow of light emission, the EL panel can use a commondrive method to drive blue, green and red pixels, without a need for aseparate burning method matching with each color.

[0021] For the emissions of red, green and blue light, light emissionhaving a maximum wavelength in at least the wavelength region of 600 to700 nm is referred to as red light, light emission having a maximumwavelength in at least the wavelength region of 500 to 600 nm isreferred to as green light, and light emission having a maximumwavelength in at least the wavelength region of 400 to 500 nm isreferred to as blue light.

[0022] Examples of the alkaline earth thioaluminates, alkaline earthaluminates, alkaline earth thiogallates, alkaline earth gallates,alkaline earth indates, and alkaline earth thioindates used in ELphosphor thin films include A₅B₂C₈, A₄B₂C₇, A₂B₂C₇, AB₂C₄, AB₄C₇,A₄B₁₄C₂₅, AB₈C₁₃, and AB₁₂Cl₉, etc. wherein A is an alkaline earthelement, B is aluminum (Al), gallium (Ga) or indium (In) and C is sulfuror oxygen. The matrix material may use these compounds alone or inadmixture of two or more and take an amorphous state where a distinctcrystalline structure is absent.

[0023] The alkaline earth element is selected from Be, Mg, Ca, Sr, Baand Ra. Of these, Mg, Ca, Sr and Ba are preferred, with Ba and Sr beingespecially preferred.

[0024] The element to be combined with the alkaline earth element is Al,Ga or In, and any desired combination is possible.

[0025] The phosphor thin film further contains sulfur and oxygen in thematrix material and preferably has the following compositional formula:

A_(x)B_(y)O_(z)S_(w):R

[0026] wherein A is at least one element selected from the groupconsisting of Mg, Ca, Sr, Ba and rare earth elements; B is at least oneelement selected from Al, Ga and In; and R is an element serving as aluminescence center and essentially containing Eu.

[0027] In the above formula, x, y, z and w denote molar ratios ofelements A, B, O and S, respectively, and are preferably in the rangesof x=0 to 5, y=0 to 15, z=0 to 30, and w=0 to 30, and more preferably,x=1 to 5, y =1 to 15, z=3 to 30, and w=3 to 30.

[0028] Oxygen is contained in the alkaline earth sulfide matrixmaterial, preferably in such amounts that the atomic ratio of oxygen tosulfur in the matrix material, as expressed by O/(S+O), is in the rangefrom 0.01 to 0.85, and especially from 0.05 to 0.5. Differently stated,the value of z/(z+w) in the formula is preferably in the range of 0.01to 0.85, more preferably 0.05 to 0.5, even more preferably 0.1 to 0.4,and especially 0.2 to 0.3.

[0029] The composition of the phosphor thin film can be ascertained byx-ray fluorescence analysis (XRF), x-ray photoelectron spectroscopy(XPS) or the like.

[0030] Oxygen is effective for outstandingly enhancing theelectroluminescent luminance of phosphor thin films. The light emittingdevice has a lifetime in that the luminance drops with the lapse oflight emitting time. The addition of oxygen improves the lifetimeperformance and prevents the luminance from dropping. The addition ofoxygen to sulfide promotes crystallization of the matrix material duringfilm deposition or during post treatment such as annealing after filmdeposition, and permits the rare earth element added to undertakeeffective transition within the compound crystal field, producing stablelight emission at a high luminance. As compared with the matrix materialof pure sulfide, the matrix material having oxygen added thereto isstable in air. This is presumably because the stable oxide componentprotects the sulfide component in the film from the ambient air.

[0031] In an alternative embodiment, the EL phosphor thin films are madeof oxides. The oxides have an improved emission life and environmentalresistance.

[0032] The oxides preferably have the following compositional formula:

A_(x)B_(y)O_(z):R

[0033] wherein A is at least one element selected from the groupconsisting of Mg, Ca, Sr, Ba and rare earth elements; B is at least oneelement selected from Al, Ga and In; and R is an element serving as aluminescence center and essentially containing Eu.

[0034] In the above formula, x, y and z denote molar ratios of elementsA, B and O, respectively, and are preferably in the ranges of x=0 to 5,y=0 to 15, and z=0 to 30, and more preferably, x=1 to 5, y=1 to 15, andz=3 to 30.

[0035] Of the phosphor thin films mentioned above, the phosphor thinfilm for blue light is preferably made of Ba_(x)Al_(y)O_(z)S_(w):Eu.Oxides wherein w=0 are also preferable, with Ca_(x)Al_(y)O_(z):Eu beingespecially preferred.

[0036] The phosphor thin film for green light is most preferably made ofSr_(x)Ga_(y)O_(z)S_(w):Eu. Oxides wherein w=0 are also preferable, withSr_(x)Al_(y)O_(z):Eu being especially preferred.

[0037] The phosphor thin film for red light is preferably made of amatrix material of alkaline earth indate having Eu added as theluminescence center, or a matrix material of alkaline earth sulfidehaving Eu added as the luminescence center. Oxides wherein w=0 astypified by Ga₂O₃ are also preferable.

[0038] The alkaline earth element is selected from Be, Mg, Ca, Sr, Baand Ra. Of these, Mg, Ca, Sr and Ba are preferred. In the case ofsulfides wherein z=0, Ca is preferred while mixtures of two or more suchas mixtures of Ca+Sr or Ca+Mg are also acceptable. In the case of oxideswherein w=0, Mg is preferred.

[0039] No particular limits are imposed on the thickness of the phosphorthin film. However, too thick a film requires an increased drive voltagewhereas too thin a film results in a low emission efficiency.Illustratively, the phosphor thin film is preferably about 100 to 2,000nm thick, especially about 150 to 700 nm although the thickness variesdepending on the identity of phosphor material.

[0040] An appropriate amount of Eu added as the luminescence center is0.1 to 10 at % based on the alkaline earth atoms. For CaS, anappropriate amount of Eu added is 0.1 to 0.5 at %, and most preferably0.2 to 0.4 at %. The element added as the luminescence center mustcontain Eu according to the invention. Eu may be added alone or incombination with one or more other elements. For example, the additionof Cu or Ce to Eu as the luminescence center can improve the responseand luminance of light emission.

[0041] A red phosphor thin film may have a thickness of about 50 to 300nm, and preferably about 150 to 250 nm. Too thick a film may require anincreased drive voltage and adversely affect the response, takingseveral seconds to several tens of seconds until emission. Too thin afilm may result in a low emission efficiency. A film thickness in theabove range ensures that an EL device is improved in both the responseand luminance of light emission.

[0042] In a preferred embodiment, red, green and blue phosphor thinfilms each have a structure of ZnS thin film/phosphor film/ZnS thinfilm. As long as the phosphor thin film is thin, the sandwiching betweenZnS thin films is effective for improving the electric charge injectionand withstand voltage of the phosphor thin film, resulting in an ELdevice capable of emitting light at a high luminance. This is trueespecially when CaS:Eu is used as the phosphor thin film, providing ared EL thin film with a high luminance and good response. The ZnS thinfilm may have a thickness of about 30 to 400 nm, and preferably about100 to 300 nm.

[0043] In another preferred embodiment, red, green and blue phosphorthin films each may have a structure of ZnS thin film/phosphor thinfilm/ZnS thin film/phosphor thin film/ZnS thin film, or a multilayerstructure of ZnS thin film/phosphor thin film/ZnS thinfilm/(repeated)/phosphor thin film/ZnS thin film.

[0044] Such phosphor thin films are preferably prepared, for example, bythe following evaporation process.

[0045] An alkaline earth sulfide having Eu added is prepared. In avacuum chamber, the source is evaporated by irradiating electron beams.By the EB evaporation of this source alone or together with theevaporation of thioaluminate, thiogallate or thioindate by resistiveheating, Eu-doped alkaline earth sulfide, alkaline earth thiogallate,alkaline earth thioaluminate or alkaline earth thioindate is formed. Thecomposition is reached by adjusting the power to the respective sources.H₂S gas may be introduced during evaporation.

[0046] Eu added to the source substance may take the form of metal,fluoride, oxide or sulfide. Since the amount of Eu added variesdepending on the source substance and the thin film to be deposited, thecomposition of the source substance is adjusted so as to achieve anappropriate dosage.

[0047] During the evaporation, the temperature of the substrate may beat room temperature to 600° C., preferably 300 to 500° C. If thesubstrate temperature is too high, the thin film of matrix material mayhave more asperities on its surface and contain pin holes therein,giving rise to the problem of current leakage on EL devices. Also thethin film can be colored brown. For this reason, the aforementionedtemperature range is preferable. The film deposition is preferablyfollowed by annealing. The preferred annealing temperature is 600 to1,000° C., and more preferably about 600 to 800° C.

[0048] The oxide phosphor thin film thus formed is preferably a highlycrystalline thin film. Crystallinity can be evaluated by x-raydiffraction, for example. To promote crystallinity, the substratetemperature is set as high as possible. It is also effective to annealthe thin film in vacuum, N₂, Ar, air, sulfur vapor or H₂S after itsformation.

[0049] No particular limits are imposed on the thickness of the lightemitting layer. However, too thick a layer requires an increased drivevoltage whereas too thin a layer results in a low emission efficiency.Illustratively, the light emitting layer is preferably about 100 to2,000 nm thick, especially about 150 to 700 nm although the thicknessvaries depending on the identity of phosphor material.

[0050] The pressure during evaporation is preferably 1.33×10⁻⁴ to1.33×10 −1 Pa (1×10⁻⁶ to 1×10⁻³ Torr). When a gas such as H₂S isintroduced, the pressure may be adjusted to 6.65×10⁻³ to 6.65×10⁻² Pa(5×10⁻⁵ to 5×10⁻⁴ Torr). If the pressure exceeds the range, theoperation of the electron gun becomes unstable, and composition controlbecomes very difficult. The rate of gas feed is preferably 5 to 200standard cubic centimeters per minute (SCCM), especially 10 to 30 SCCMalthough it varies depending on the capacity of the vacuum system.

[0051] If desired, the substrate may be moved or rotated duringevaporation. By moving or rotating the substrate, the deposited filmbecomes uniform in composition and minimized in the variation ofthickness distribution.

[0052] When the substrate is rotated, the number of revolutions ispreferably at least about 10 rpm, more preferably about 10 to 50 rpm,and especially about 10 to 30 rpm. If the rotational speed of thesubstrate is too high, there may arise a problem of seal upon admissioninto the vacuum chamber. If the rotational speed of the substrate is toolow, compositional gradation may occur in the thickness direction withinthe chamber so that the resulting light emitting layer may have poorcharacteristics. The means for rotating the substrate may be anywell-known rotating mechanism including a power source such as a motoror hydraulic rotational mechanism and a power transmission/gearmechanism having a combination of gears, belts, pulleys and the like.

[0053] The means for heating the evaporation source and the substratemay be selected, for example, from tantalum wire heaters, sheath heatersand carbon heaters, as long as they have the predetermined thermalcapacity, reactivity or the like. The temperature reached by the heatingmeans is preferably in the range of about 100 to about 1,400° C., andthe precision of temperature control is about ±1° C., preferably about±0.5° C. at 1,000° C.

[0054]FIG. 1 illustrates one exemplary construction of the apparatus forforming the light emitting layer according to the invention. Referenceis made to an embodiment wherein Eu-doped alkaline earth sulfide,alkaline earth thiogallate, alkaline earth thioaluminate or alkalineearth thioindate is produced by using Eu-added alkaline earth sulfideand any one of thiogallate, thioaluminate and thioindate as theevaporation sources and admitting H₂S during evaporation. In theillustrated embodiment, a substrate 12 on which the light emitting layeris to be deposited, a resistive heating evaporation source in the formof a Knudsen cell 14 and an EB evaporation source 15 are disposed withina vacuum chamber 11.

[0055] In the resistive heating evaporation source or K cell 14 servingas means for evaporating alkaline earth sulfide, an alkaline earthsulfide 14 a having a luminescence center added thereto is contained.The K cell 14 is heated by a heater (not shown) so that the metalmaterial may evaporate at a desired evaporation rate.

[0056] The electron beam (EB) evaporation source 15 serving as means forevaporating thioaluminate, thiogallate or thioindate include a crucible50 which contains thioaluminate, thiogallate or thioindate 15 a and anelectron gun 51 having an electron emitting filament 51 a built therein.Built in the electron gun 51 is a mechanism for controlling an electronbeam. To the electron gun 51 are connected an ac power supply 52 and abias power supply 53. The electron gun 51 produces an electron beam at apredetermined power in a controlled manner, for evaporating thethioaluminate, thiogallate or thioindate 15 a at a predetermined rate.Although the evaporation source is controlled by the K cell and electrongun in the illustrated embodiment, multi-source simultaneous evaporationusing a single electron gun is also possible. The evaporation process ofthe latter is known as multisource pulse evaporation process.

[0057] In the illustrated embodiment, the evaporation sources 14 and 15are depicted, for the convenience of illustration, at positionscorresponding to discrete local areas of the substrate. Actually, theevaporation sources are located such that the deposited film may becomeuniform in composition and thickness.

[0058] The vacuum chamber 11 has an exhaust port 11 a through which thechamber is evacuated to establish a predetermined vacuum in the chamber.The vacuum chamber 11 also has an inlet port 11 b through which areactant gas such as hydrogen sulfide is admitted into the chamber.

[0059] The substrate 12 is fixedly secured to a holder 12 a. The holder12 a has a shaft 12 b which is rotatably held by an outside rotatingshaft mount (not shown) so that the vacuum may be maintained in thechamber 11. The shaft 12 b is adapted to be rotated at a predeterminednumber of revolutions by a rotating means (not shown). A heating means13 in the form of a heater wire is closely secured to the substrateholder 12 a so that the substrate may be heated and maintained at thedesired temperature.

[0060] Using the illustrated apparatus, the vapor of alkaline earthsulfide and the vapor of thioaluminate, thiogallate or thioindate areevaporated from the K cell 14 and EB evaporation source 15 and depositedon the substrate 12 where they are bound together to form a fluorescentlayer of Eu-doped alkaline earth sulfide, alkaline earth thiogallate,alkaline earth thioaluminate or alkaline earth thioindate. By rotatingthe substrate 12 during the evaporation process if desired, the lightemitting layer being deposited can be made more uniform in compositionand thickness distribution.

[0061] Using the phosphor thin film of the invention as a light emittinglayer 3, an inorganic EL device is manufactured, for example, to thestructure shown in FIG. 2.

[0062]FIG. 2 is a partially cross-sectional, perspective view showing anexemplary construction of the inorganic EL device using the lightemitting layer of the invention. In FIG. 2, a predetermined pattern oflower electrodes 5 is formed on a substrate 1, and a first thickinsulating layer (or thick-film dielectric layer) 2 is formed on thelower electrodes 5. On the first insulating layer 2, a light-emittinglayer 3 and a second insulating layer (or thin-film dielectric layer) 4are successively formed. On the second insulating layer 4, apredetermined pattern of upper electrodes 6 is formed so as to constructa matrix circuit with the lower electrodes 5. The red, green or bluephosphor thin film is selectively coated at the intersections of matrixelectrodes.

[0063] Between two adjacent ones of the substrate 1, electrodes 5, 6,thick-film insulating layer 2 and thin-film insulating layer 4, anintermediate layer such as a bond enhancing layer, stress relief layeror reaction preventing barrier layer may be disposed. The thick film maybe improved in smoothness as by polishing its surface or using asmoothing layer.

[0064] Preferably, a BaTiO₃ thin-film layer is formed as the barrierlayer between the thick-film insulating layer and the thin-filminsulating layer.

[0065] Any desired material may used as the substrate as long as thesubstrate has a heat resistant temperature or melting point of at least600° C., preferably at least 700° C., especially at least 800° C. sothat the substrate may withstand the thick-film forming temperature, theforming temperature of the EL fluorescent layer and the annealingtemperature of the EL device, the substrate allows deposition thereon offunctional thin films such as a light emitting layer by which the ELdevice can be constructed, and the substrate maintains the predeterminedstrength. Illustrative examples include glass substrates, ceramicsubstrates of alumina (Al₂O₃), forsterite (2MgO·SiO₂), steatite(MgO·SiO₂), mullite (3Al₂O₃·2SiO₂), beryllia (BeO), aluminum nitride(AlN), silicon nitride (SiN), and silicon carbide (SiC+BeO) as well asheat resistant glass substrates of crystallized glass or the like. Ofthese, alumina substrates and crystallized glass substrates areespecially preferable. Where heat transfer is necessary, beryllia,aluminum nitride, silicon carbide and the like are preferred.

[0066] Also useful are quartz, heat oxidized silicon wafers, etc. aswell as metal substrates such as titanium, stainless steel, Inconel andiron base materials. Where electro-conductive substrates such as metalsubstrates are used, a structure in which a thick film having aninternal electrode is formed on a substrate is preferred.

[0067] Any well-known thick-film dielectric material may be used as thethick-film dielectric material (first insulating layer). Materialshaving a relatively high permittivity are preferred.

[0068] For example, lead titanate, lead niobate and barium titanatebased materials can be used.

[0069] The dielectric thick film has a resistivity of at least 10⁸ Ω·cm,especially about 10¹⁰ to 10¹⁸ Ω·cm. A material having a relatively highpermittivity as well is preferred. The permittivity ε is preferablyabout 100 to 10,000. The preferred thickness is 5 to 50 μm, especially10 to 30 μm.

[0070] The insulating layer thick film is formed by any desired method.Methods capable of relatively easily forming films of 10 to 50 μm thickare useful, and the sol-gel method and printing/firing method areespecially preferred.

[0071] Where the printing/firing method is employed, a material isfractionated to an appropriate particle size and mixed with a binder toform a paste having an appropriate viscosity. The paste is applied ontoa substrate by a screen printing technique, and dried. The green sheetis fired at an appropriate temperature, yielding a thick film.

[0072] Examples of the material of which the thin-film insulating layer(second insulating layer) is made include silicon oxide (SiO₂), siliconnitride (SiN), tantalum oxide (Ta₂O₅), strontium titanate (SrTiO₃),yttrium oxide (Y₂O₃), barium titanate (BaTiO₃), lead titanate (PbTiO₃),PZT, zirconia (ZrO₂), silicon oxynitride (SiON), alumina (Al₂O₃), leadniobate, PMN-PT base materials, and multilayer or mixed thin films ofany. In forming the insulating layer from these materials, any ofconventional methods such as evaporation, sputtering, CVD, sol-gel andprinting/firing methods may be used. The insulating layer preferably hasa thickness of about 50 to 1,000 nm, especially about 100 to 500 nm.

[0073] The electrode (lower electrode) is formed at least on thesubstrate side or within the first dielectric layer. As the electrodelayer which is exposed to high temperature during formation of a thickfilm and during heat treatment along with the light emitting layer, usemay be made of a customary metal electrode containing as a maincomponent one or more elements selected from palladium, rhodium,iridium, rhenium, ruthenium, platinum, tantalum, nickel, chromium andtitanium.

[0074] Another electrode layer serving as the upper electrode ispreferably a transparent electrode which is transmissive to light in thepredetermined emission wavelength region because the emitted light oftenexits from the opposite side to the substrate. When the substrate andinsulating layer are transparent, a transparent electrode may also beused as the lower electrode because this permits the emitted light toexit from the substrate side. Use of transparent electrodes of ZnO, ITOor the like is especially preferred. ITO generally contains In₂O₃ andSnO in stoichiometry although the oxygen content may deviate somewhattherefrom. An appropriate proportion of SnO₂ mixed with In₂O₃ is about 1to 20%, more preferably about 5 to 12% by weight. For IZO, anappropriate proportion of ZnO mixed with In₂O₃ is generally about 12 to32% by weight.

[0075] Also the electrode may be a silicon-based one. The siliconelectrode layer may be either polycrystalline silicon (p-Si) oramorphous silicon (a-Si), or even single crystal silicon if desired.

[0076] In addition to silicon as the main component, the electrode isdoped with an impurity for imparting electric conductivity. Any dopantmay be used as the impurity as long as it can impart the desiredconductivity. Use may be made of dopants commonly used in the siliconsemiconductor art. Exemplary dopants are B, P, As, Sb, Al and the like.Of these, B, P, As, Sb and Al are especially preferred. The preferreddopant concentration is about 0.001 to 5 at %.

[0077] In forming the electrode layer from these materials, any ofconventional methods such as evaporation, sputtering, CVD, sol-gel andprinting/firing methods may be used. In forming a structure in which athick film having an internal electrode is formed on a substrate, thesame method as used in forming the dielectric thick film is preferred.

[0078] The electrode layer should preferably have a resistivity of up to1 Ω·cm, especially about 0.003 to 0.1 Ω·cm in order to apply aneffective electric field across the light emitting layer. The preferredthickness of the electrode layer is about 50 to 2,000 nm, especiallyabout 100 to 1,000 nm although it depends on the electrode material.

[0079] The EL panel of the invention has been described while it can beapplied to other forms of display device, typically full-color panels,multicolor panels and partial color panels partially displaying threecolors.

EXAMPLE

[0080] Examples are given below for illustrating the invention in moredetail.

Example 1

[0081] An EL panel according to the invention was fabricated. For thesubstrate and thick-film insulating layer, BaTiO₃ base dielectricmaterial having a permittivity of 5,000 was commonly used. For the lowerelectrode, a Pd electrode was used. On fabrication, a sheet of thesubstrate was formed, and the lower electrode and thick-film insulatinglayer were screen printed thereon to form a green sheet, which wasco-fired. The surface was polished, obtaining a substrate bearing athick-film first insulating layer of 30 μm thick. On this substrate, aBaTiO₃ coating was formed by sputtering as a buffer layer to 400 nm.This was annealed in air at 700° C., obtaining a composite substrate.

[0082] On the composite substrate, phosphor thin films of three types,red, green and blue each were formed as a structure of Al₂O₃ film (50nm)/ZnS film (200 nm)/phosphor thin film or light emitting layer (300nm)/ZnS film (200 nm)/Al₂O₃ film (50 nm) in order that the resulting ELdevice produce stable light emission.

[0083] To form the phosphor thin film of each color at predeterminedsites, a masking pattern was previously furnished for each color, andeach film was partially formed by masked evaporation.

[0084] For the phosphor thin films of three types, red, green and blue,CaS, SrGs₂S₄ and BaAl₂S₄ base phosphor thin films were used,respectively. In every film, Eu was used as the luminescence center.

[0085] The red phosphor thin film was prepared by the followingprocedure. Used for this film formation was an apparatus as shown inFIG. 1 wherein only one electron gun was used.

[0086] An EB source 15 loaded with CaS powder having 0.5 mol % Eu addedwas placed in a vacuum chamber 11 into which H₂S gas was admitted. TheCaS was evaporated from the source and deposited on a rotating substrateheated at 400° C., forming a thin film. The evaporation rate of thesource was adjusted such that the film was deposited on the substrate ata deposition rate of 1 nm/sec. The H₂S gas was fed at 20 SCCM. In thisway, a phosphor thin film was formed. Specifically the thin film wasobtained as the structure of Al₂O₃ film (50 nm)/ZnS film (200nm)/phosphor thin film (300 nm)/ZnS film (200 nm)/Al₂O₃ film (50 nm).The structure was annealed in air at 750° C. for 10 minutes.

[0087] Similarly, a phosphor thin film was formed on a Si substrate. Theresulting phosphor thin film in the form of CaS:Eu thin film wasanalyzed for composition by fluorescent x-ray analysis, finding anatomic ratio of Ca:S:Eu=24.07:25.00:0.15.

[0088] The green phosphor thin film was prepared by the followingprocedure. Used for this film formation was an apparatus as shown inFIG. 1 wherein one electron gun and one resistive heating evaporationsource (cell) were used.

[0089] An EB source 15 loaded with SrS powder having 5 mol % Eu addedand a resistive heating source 14 loaded with Ga₂S₃ powder were placedin a vacuum chamber 11 into which H₂S gas was admitted. The reactantswere evaporated from the respective sources and deposited on a rotatingsubstrate heated at 400° C., forming a thin film. The evaporation ratesof the respective sources were adjusted such that the film was depositedon the substrate at a deposition rate of 1 nm/sec. The H₂S gas was fedat 20 SCCM. In this way, a phosphor thin film was formed. Specificallythe thin film was obtained as the structure of Al₂O₃ film (50 nm)/ZnSfilm (200 nm)/phosphor thin film (300 nm)/ZnS film (200 nm)/Al₂O₃ film(50 nm). The structure was annealed in air at 750° C. for 10 minutes.

[0090] Similarly, a phosphor thin film was formed on a Si substrate. Theresulting phosphor thin film in the form of Sr_(x)Ga_(y)O_(z)S_(w):Euthin film was analyzed for composition by fluorescent x-ray analysis,finding an atomic ratio of Sr:Ga:O:S:Eu=6.02:19.00:11.63:48.99:0.34.

[0091] The blue phosphor thin film was prepared by the followingprocedure. Used for this film formation was an apparatus as shown inFIG. 1 wherein one electron gun and one resistive heating evaporationsource (cell) were used.

[0092] An EB source 15 loaded with BaS powder having 5 mol % Eu addedand a resistive heating source 14 loaded with Al₂S₃ powder were placedin a vacuum chamber 11 into which H₂S gas was admitted. The reactantswere evaporated from the respective sources and deposited on a rotatingsubstrate heated at 400° C., forming a thin film. The evaporation ratesof the respective sources were adjusted such that the film was depositedon the substrate at a deposition rate of 1 nm/sec. The H₂S gas was fedat 20 SCCM. In this way, a phosphor thin film was formed. Specificallythe thin film was obtained as the structure of Al₂O₃ film (50 nm)/ZnSfilm (200 nm)/phosphor thin film (300 nm)/ZnS film (200 nm)/Al₂O₃ film(50 nm). The structure was annealed in air at 750° C. for 10 minutes.

[0093] Similarly, a phosphor thin film was formed on a Si substrate. Theresulting phosphor thin film in the form of Ba_(x)Al_(y)O_(z)S_(w):Euthin film was analyzed for composition by fluorescent x-ray analysis,finding an atomic ratio of Ba:Al:O:S:Eu=8.91:18.93:9.33:28.05:0.35.

[0094] By RF magnetron sputtering technique using an ITO oxide target, atransparent ITO electrode of 200 nm thick was formed on the resultingstructure at a substrate temperature of 250° C. The electrode waspatterned to complete an EL device of the matrix structure.

[0095] An electric field having a frequency of 240 Hz and a pulse widthof 50 μS at seven different voltages was applied to the two electrodesof each matrix in the EL panel, providing each color with 8 bitgradation. The EL panel emitted light in 512 colors at an averageluminance of 100 cd/m² and with a good response.

Example 2

[0096] The procedure of Example 1 was repeated using a SrAl₂O₄:Eu thinfilm as the green phosphor instead of the Sr_(x)Ga_(y)O_(z)S_(w):Eu thinfilm. Substantially equivalent results were obtained.

[0097] The EL panel of the invention employs red, green and bluephosphor thin film materials capable of light emission of a good colorpurity and high luminance without a need for filters, and differentlystated, phosphor matrix materials having analogous chemical or physicalproperties and doped with Eu as the luminescence center commonly for allthree colors. This leads to the advantages of simplified driving of afull color EL panel, minimized luminance variation, increasedmanufacturing yield, and reduced manufacturing cost of the panelincluding circuitry. The invention is of great commercial worth.

[0098] There has been described an EL panel comprising phosphor thinfilms eliminating a need for RGB phosphor filters, having a satisfactorycolor purity and best suited for driving RGB in full-color EL display.

[0099] Japanese Patent Application No. 2001-228272 is incorporatedherein by reference.

What is claimed is:
 1. An EL panel comprising EL phosphor thin films of three types which emit red, green and blue light, respectively, the EL phosphor thin films of three types commonly and essentially containing europium as a luminescence center.
 2. The EL panel of claim 1 wherein the EL phosphor thin films of three types have the compositional formula: A_(x)B_(y)O_(z)S_(w):R wherein A is at least one element selected from the group consisting of Mg, Ca, Sr, Ba and rare earth elements, B is at least one element selected from the group consisting of Al, Ga and In, x is in the range of 0 to 5, y is in the range of 0 to 15, z is in the range of 0 to 30, w is in the range of 0 to 30, and R is an element serving as the luminescence center and essentially containing europium.
 3. The EL panel of claim 1 wherein the EL phosphor thin film which emits red light is made of a matrix material comprising an alkaline earth sulfide, the EL phosphor thin film which emits green light is made of a matrix material comprising an alkaline earth thiogallate, and the EL phosphor thin film which emits blue light is made of a matrix material comprising an alkaline earth thioaluminate.
 4. The EL panel of claim 3 wherein the alkaline earth sulfide is calcium sulfide.
 5. The EL panel of claim 3 wherein the alkaline earth thiogallate is strontium thiogallate.
 6. The EL panel of claim 3 wherein the alkaline earth thioaluminate is barium thioaluminate.
 7. The EL panel of claim 1 wherein the EL phosphor thin films of three types which emit red, green and blue light, respectively, each comprise an oxysulfide obtained by incorporating oxygen in at least one compound selected from the group consisting of an alkaline earth sulfide, alkaline earth thioaluminate, alkaline earth thiogallate, and alkaline earth thioindate, and the molar ratio of oxygen element to sulfur element in said oxysulfide, as expressed by O/(S+O), is in the range between 0.01 and 0.85.
 8. The EL panel of claim 1 wherein any one of the EL phosphor thin films of three types which emit red, green and blue light, respectively, is made of an oxide. 